An electric power steering apparatus (EPS) which provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel (handle) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the calculated current command value. A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor which is connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MPU (Micro Processing Unit) and an MCU (Micro Controller Unit))), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a torque control section 31. The torque control section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vel using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited the maximum value thereof at a current limiting section 33. The current command value Irefm limited the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result I (=Irefm−Im) at the subtracting section 32B is current-controlled at the current control section 35 such as a proportional-integral (PI) control and so on. The voltage control value Vref obtained by the current control is inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a self-aligning torque (SAT) detected or estimated and an inertia compensation value 342 at an adding section 344. The addition result is further added with a convergence control value 341 at an adding section 345. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the control characteristics.
In a case that the motor 20 is a three-phase brushless motor, details of the PWM-control section 36 and the inverter 37 have a configuration as shown in FIG. 3, and the PWM-control section 36 comprises a duty calculating section 36A that calculates the duties D1 to D6 which are used in a three-phase PWM-control by using the voltage control value Vref in accordance with a predetermined equation, and a gate driving section 36B that drives gates of the FETs serving as the driving device by means of the duties D1 to D6 and switches-ON or switches-OFF for compensating a dead time. The modulation signal (carrier) CF is inputted into the duty calculating section 36A, and the duty calculating section 36A calculates the duties D1 to D6 by synchronized to the modulation signal CF. The inverter 37 is configured to the three-phase bridges of the FETs (an upper-stage is FET1 to FET3, and a lower-stage is FET4 to FET6). The motor 20 is driven by switching-ON or switching-OFF the respective FETs by using the duties D1 to D6.
In this example, the duties D1 to D3 control the switching-ON or the switching-OFF of the upper-stage FET1 to FET3, and the duties D4 to D6 control the switching-ON or the switching-OFF of the lower-stage FET4 to FET6.
A motor release switch 23 is interposed between the inverter 37 and the motor 20 in order to block a current supply when the assist control is stopped or the like. The motor release switch 23 comprises the FETs with a parasitic diode disposed to respective phases.
In such an electric power steering apparatus, a conventional technology, which performs a control by means of only a commutation process with a simple two-phase/three-phase conversion in response to a torque command of the motor and an electrical angle of the motor, cannot control the heat generation of the respective FETs, individually. Thus, even though the heat generation amounts of the overall ECU, each site of the ECU, the motor or the like are individually measured and estimated, in final, by limiting the torque command value of the motor, a protection process which suppresses the heat generation in the overall system can only be performed. For example, when the maximum current in the system flows through the system in a state of a steering holding of the handle, the heat generation is concentrated in a particular FET. In a case that the above state continues in a long time, the FET, which the heat generation is concentrated, is earlier deviated from an operating temperature range than other parts, and then is failed.
In order to prevent the above failure, the conventional technique monitors the system by individually measuring or estimating a temperature of each site of the system and protects by suppressing the torque command value of the motor so that the temperatures of all locations do not deviate from the operating temperature range. However, since such a protecting process limits the torque command value of the motor, there is a problem that the limitation must be exactly controlled such that a decreasing of the assist force and a distinct lack of the assist do not occur or a driver does not notice the decreasing of the assist force.
As resolving such a problem, Japanese Unexamined Patent Publication No. 2004-161118 A (Patent Document 1) proposes a power steering apparatus that suppresses the occurrence of the specific noise in the PWM-control and prevents from increasing a temperature of the driving device up to a temperature in a failure of the motor. The driving control of the motor is accomplished by PWM-controlling the driving circuit by the electronic control unit. The electronic control unit changes a carrier frequency of the PWM signal which is applied to the driving circuit in response to the temperature of the power driving device detected by the driving device temperature sensor. That is, when the temperature of the power driving device is lower than a predetermined frequency switching temperature, the PWM frequency is set to a predetermined high frequency, and when the temperature of the power driving device is higher than or equal to the predetermined frequency switching temperature, the PWM frequency is set to a predetermined low frequency.
Further, a motor driving unit disclosed in Japanese Unexamined Patent Publication No. 2004-88888 A (Patent Document 2) introduces a driving current from two among a U-phase terminal, a V-phase terminal and a W-phase terminal in order to reduce the heat generation due to a switching loss. When the driving current should be derived from another terminal, a target voltage value of the terminal from which the driving current is derived, is set to a ground level of an electric potential. When the driving current is introduced from one terminal in the U-phase terminal to the W-phase terminal and the driving current should be derived from other two terminals, the target voltage value of terminal which the driving current is introduced, is set to the output voltage of the DC (direct current)-power supply.